Controlled chromatic aberration techniques may be utilized for distance sensing metrology applications. As described in “Pseudocolor Effects of Longitudinal Chromatic Aberration,” G. Molesini and S. Quercioli, J. Optics (Paris), 1986, 17(6):279-282, controlled longitudinal chromatic aberration (also referred to herein as axial chromatic dispersion) may be introduced in an optical imaging system, causing the imaging system focal length to vary with wavelength, which provides means for optical metrology. In particular, a lens can be designed whose back focal length (BFL) is a monotonic function of wavelength. In white light operation such a lens exhibits a rainbow of axially dispersed foci that can be used as a spectral probe for distance sensing applications.
As a further example, U.S. Pat. No. 7,477,401, which is hereby incorporated herein by reference in its entirety, discloses that an optical element having axial chromatic aberration, also referred to as axial or longitudinal chromatic dispersion, may be used to focus a broadband light source such that the axial distance to the focus varies with the wavelength. Thus, only one wavelength will be precisely focused on a surface, and the axial distance or height of the surface determines which wavelength is best focused. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole and/or the end of an optical fiber. Upon reflection from a surface, only the wavelength that is well-focused on the surface is well-focused on the pinhole and/or fiber. All of the other wavelengths are poorly focused on the fiber, and so will not couple much power into the fiber. Therefore, the signal level will be greatest for the wavelength corresponding to the height of the object. A spectrometer at the detector measures the signal level for each wavelength, which effectively indicates the height of the object.
Certain manufacturers refer to a practical and compact optical assembly that is suitable for chromatic confocal ranging in an industrial setting as a chromatic confocal point sensor and/or as an “optical pen.” One example of optical pen instruments that measure Z height are those manufactured by STIL, S. A. of Aix-en-Provence, France (STIL S. A.). As a specific example, the STIL optical pen model number OP 300NL measures Z heights and has a 300 micron range.
Another configuration for a chromatic confocal point sensor and optical pen is described in commonly assigned U.S. Pat. No. 7,626,705 (the '705 patent) which is hereby incorporated herein by reference in its entirety. The '705 patent discloses a lens configuration providing an improved optical throughput and an improved spot size which results in improved measurement resolution in comparison with various commercially available configurations.
One figure of merit for an optical pen is its range-to-resolution ratio, for example a versatile optical pen should be able to measure over a long range with high resolution. Generally speaking, in known optical pens, a relatively large numerical aperture is necessary for high resolution measurements, and to extend the measuring range for a given numerical aperture, a larger pen diameter is required. However, it is desirable to have a compact design for a chromatic confocal point sensor optical pen in various applications. Thus, an optical pen having an improved range-to-resolution ratio in a compact size would be desirable.